In the oil and gas industry seismic sensors are deployed at various locations, such as on the earth surface, in the sea, at the seabed, or in a borehole, to provide operationally significant subsurface structural and material information by measuring seismic signals reflected from changes in the subsurface structures. In this, seismic sensors are commonly used for purposes of obtaining useful data relating to acoustic impedance contrasts in subsurface structures.
In seismic signal detection, the vibrations in the earth resulting from a source of seismic energy are sensed at discrete locations by sensors, and the output of the sensors used to determine the structure of the underground formations. The source of seismic energy can be natural, such as earthquakes and other tectonic activity, subsidence, volcanic activity or the like, or man-made such as acoustic signals from surface or underground operations, or from deliberate operation of seismic sources at the surface or underground. For example, the sensed seismic signals may be direct signals that are derived from micro-seismicity induced by fracturing or reservoir collapse or alteration, or reflected signals that are derived from an artificial source of energy. Sensors fall into two main categories; hydrophones which sense the pressure field resulting from a seismic source, or geophones which sense vibration arising from a seismic source.
Typically, geophones are sensitive to vibrations of low or very low frequency. As depicted in FIG. 1B, a typical geophone 10 has one or more cylindrical moving coil 12 that is suspended by springs 20, 22 so as to be disposed around a magnet 15 having pole pieces 16, 18. The geophone 10 has a housing 24 and end caps 26. Each moving coil 12 is maintained at a neutral, rest position by the springs 20, 22, and is free to oscillate in a magnetic field of the magnet 15 from a centered position thereof. Springs 20, 22 are designed to maintain the coil 12 at a centered, equilibrium position relative to the magnetic field of the magnet 15. Note again FIG. 1B.
When the earth moves due to the seismic energy propagating either directly from the source or via an underground reflector, the geophone, which can be located at the earth's surface, in the sea or at the seabed, or on the wall of a borehole which penetrates the earth, moves with the particle motion caused by acoustic wave propagation. If the axis of the geophone is aligned with the direction of motion, however, the moving coil mounted on the spring inside the geophone stays in the same position causing relative motion of the coil with respect to the housing. When the coil moves in the magnetic field, a voltage is induced in the coil which can be output as a signal.
If the geophone is tilted, i.e., is moved away from the orientation that it is designed for, the moving coil is eccentered with respect to the magnetic field in the magnet. Note FIG. 1C. For example, as depicted in FIG. 1A, typically vertical geophones are used in land seismic survey operations. The spring to support the moving coil is pre-stressed to compensate for gravitational force so that the moving coil is centered in the geophone. However, the geophones are manually planted in the ground and may not be vertical. If such a geophone is tilted, the pre-stressed spring causes the moving coil to move in the upward direction relative to the neutral position of the coil in the vertical position of the geophone, as depicted in FIG. 1C. The neutral or rest position of the moving coil is designated in FIG. 1C as x0, and the displaced position due to tilt θ is designated as x. Eventually, if the amount of tilt is large, the moving coil hits an end cap of the geophone so that the geophone is no longer able to respond to the seismic vibrations.
Although FIG. 1A depicts exemplary land seismic with typical vertical geophones, it is also possible to use three-component geophones of the type discussed herein in connection with seabed and borehole seismic.
FIG. 1D illustrates graphically the relationships between tilt of a 10 Hz vertical geophone and the geophone response parameters So, Do, and fo using measured data. In this, as evident from FIG. 1D, if a vertical geophone is tilted from its vertical position the geophone response parameters So, Do, and fo change based on the amount of tilt.
In land seismic survey operations, seismic data are processed by assuming that all the geophones that are planted on the land surface are vertical. If seismic waves propagate in the upward direction, a tilted geophone will output signal that is altered due to tilt and reduced by an amount equal to cos(θ), where θ is measured from vertical—note FIG. 1C. As a consequence, incorrect orientation of the geophones can cause misinterpretation of the formation properties, by changing the apparent amplitude of reflected waves.
In seabed seismic survey operations, an ocean bottom cable (OBC) is deployed from a boat to the seabed. Note FIGS. 2A and 2B. Seismic sensors are mounted on the side of the cable. After the cable is deployed in the sea, the orientation of the sensors may be horizontal, vertical or upside down. Gimbaled mountings may be used so that a sensor is always vertically oriented irrespective of how the sensor is deployed. Recently, three-component omni-tiltable geophones have been used in sensor packages with a tiltmeter or inclinometer. By knowing the orientation of the mounting of the three-component geophone, it is possible to rotate the axis of the seismic measurements. Ideally, a magnetometer may be employed to know the horizontal orientation of the sensor package so as to transpose the detected seismic signals in the physical earth coordinates to determine from which direction the seismic signals arrive at the seismic sensors.
In a borehole seismic survey, one or more geophone is deployed downhole in a borehole. Note FIGS. 3A and 3B. The trajectory of the borehole is usually known by independent measurement. If the length of the deploying cable is known, i.e., the along depth, it is possible to determine orientation and position of the downhole sensor package, i.e., azimuth, inclination, depth and horizontal departure from the well head. The information missing is the relative bearing of the sensor package. As used herein, “relative bearing” refers to the angle of sensor package orientation. The sensor package or sonde is typically cylindrical and may rotate in the borehole, and the orientation of the two horizontal geophones will not be known. To identify the sensor orientation, it may be possible to deploy such downhole geophones with a gyroscope. Alternatively, a tiltmeter or inclinometer may be integrated with the downhole geophones to determine relative geophone bearing against the direction of gravity.
In addition to the issues discussed above, others arise during manufacturing and assembly of geophones. In this, during manufacture the geophone moving coil may not be properly centered around the magnetic field in the magnet. After assembly, it is not possible to see whether or not the moving coil is properly centered around the magnet so as to be at its desired neutral rest position.
Displacement of the moving coil from its neutral rest position during assembly of the geophone may lead to changes in the geophone response parameters and increase in harmonic distortion. The offset of the coil reduces the dynamic range of the geophone. In a worst case, the geophone moving coil may hit the top or the bottom of the housing and therefore not respond to seismic signals that are received by the geophone. In particular, a properly centered moving coil is highly important for low frequency geophones, such as seismometers, since the acceptable operating tilt range for such geophones is small, i.e., in the order of a few degrees. Such low frequency geophones or seismometers often use a built-in carpenter's level or eye bubble to guide installation of the devices; however, such eye bubble levels show only the tilt angle of the geophone or seismometer housing relative to gravity, but do not show the eccentricity of the moving coil without a built-in displacement sensor inside the geophone or seismometer.
As previously mentioned, in the past, gimbaled geophones have been employed to avoid tilt in the geophone. However, gimbaled geophones tend to be bulky and are more expensive due to the additional hardware that is required for the gimbaled structure. Geophones with tiltmeters and other tilt determining sensors are known in the art, but require additional hardware and are difficult to fit in the limited space that is typically available in seismic surveying operations. In addition, extra wiring is required for electrical connection. Since a geophone type device is a passive sensor, only a twisted wire cable is required to connect the geophone to a data acquisition system. Typically, in land, seabed, or borehole seismic acquisition operations many geophones are connected using multi-twisted pair cables. Extra wiring for built-in tilt sensors means that additional conductors must be added to the cables thereby increasing cable weight and cost, and the maintenance costs for the cables. In addition, larger connectors are required which poses a reliability issue. For seabed and borehole operations, any additional connectors or connections to the cable are perceived as unreliable due to a tendency to leak. Therefore, increased wiring is not a preferred approach in seismic operations.
For single seismic sensors having tilt accelerometers, the electronics may be located away from the sensors causing alignment errors between the seismic sensors and the tilt accelerometers. Such errors are difficult to control making the use of such configurations problematic.
Accordingly, it will be appreciated that there exists a desire to improve upon conventional methods and systems that use geophones in order to improve the accuracy of seismic measurements.
The limitations of conventional seismic sensor designs noted in the preceding are not intended to be exhaustive but rather are among many which may tend to reduce the effectiveness of previously known sensor mechanisms in field operation. The above should be sufficient, however, to demonstrate that sensor structures existing in the past will admit to worthwhile improvement.